An immersion nozzle generally requires replacement for reasons such as breaking, fracture, and durability limit resulting from wear damage caused by molten steel or clogging of an inner bore thereof caused by adhesion and buildup of inclusions contained in molten steel, such as alumina particles which are non-metal particles, and such replacement inevitably causes interruption or stop of a steel continuous casting operation. As a means to realize prolonged pouring from a viewpoint of a need for improvement in efficiency of casting operation, an apparatus designed to replace an immersion nozzle with a new one while minimizing the interruption of a steel continuous casting operation has been introduced (see, for example, the following Patent Documents 1 and 2).
An immersion nozzle for use with such an immersion nozzle replacement apparatus has a fundamental structure which can be broadly divided into two elements: a tubular-shaped nozzle body having an inner bore extending in a vertical direction and serving as a molten steel flow pathway; and a flange formed by increasing a cross-sectional area with respect to the nozzle body in a horizontal direction, in such a manner that it can be supported by a supporting device of an immersion nozzle replacement apparatus, from therebelow to allow the nozzle body to be supported and pushed upwardly against the force of gravity and brought into contact with a member located thereabove. In this fundamental structure, an interface region between the nozzle body and the flange in which a cross-sectional area of the immersion nozzle increases will hereinafter be referred to as “neck region”.
It is known that the neck region is a stress concentration point in structure, and crack can be formed due to a thermal stress and a mechanical stress applied thereto. The crack formed in the neck region poses a problem in terms of durable life of the immersion nozzle and quality of steel. When molten steel flows through the inner bore of the immersion nozzle, a pressure level in an internal space of the inner bore inclines toward a negative side, so that air is sucked through the crack formed in the neck region to cause oxidation of a carbon component constituting a refractory material. This is likely to lead to leakage of molten steel and to contamination of molten steel by oxygen.
Therefore, various proposals have heretofore been made for suppression of crack formation in the neck region as a technical problem (e.g., the following Patent Documents 3 to 6). These conventional techniques are intended to take measures from a viewpoint of a structure and shape of an immersion nozzle, and measures from a viewpoint of a material of an immersion nozzle, such as forming the nozzle body and the flange, respectively, by different materials. However, all of the measures fail to sufficiently prevent crack formation in the neck region. Because, as long as an immersion nozzle has a region in which a cross-sectional area of a nozzle body increases upwardly, i.e., has the neck region, the immersion nozzle can be deemed as a structure which is liable to be cracked when thermal and mechanical stresses are strongly applied thereto.